This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Synchrotron beamtime is requested for X-ray crystallographic studies of human DNA topoisomerase I (topo I). Topo I is an essential enzyme in DNA replication, and the target of anti-cancer drugs. The synchrotron beamtime will be used to determine structures of complexes of topo I with small molecule compounds for design and development of new topo I drugs.